1. Field of the Invention
The present invention relates generally to coating compositions, and more particularly to coating compositions that can be used for anti-fog and anti-splash applications.
2. Background of the Prior Art
For many years hydrophilic surfactants have been used as anti-fog coatings and films for windows, mirrors, eye glasses, goggles, face masks, automobile windshields, medical instruments, and numerous other substrates. Fog is caused when moisture condenses on the surface of a relatively cool, nonporous surface exposed to a warmer humid atmosphere (it is also present on porous surfaces, but not easily seen). When applied to a transparent or reflective substrate, the hydrophilic property of an anti-fog surfactant spreads condensed droplets over the surface of the substrate, thus maintaining a high degree of the dry transparency or reflectivity. Superior products dry virtually clear and invisible on the substrate, and their de-fogging property persists for a substantial period, ideally through the use cycle of the device to which they are applied; yet they are highly water soluble and flushed off quickly if wet conditions become extreme.
Anti-splash products are typically designed to keep automobile windshields clear during heavy rain or splashes. This is accomplished by applying a water-repellant (hydrophobic) agent to the windshield surface: the coating repels water and makes it rapidly run off the windshield surface, minimizing the degree to which visibility is obscured and thus reducing danger.
Several properties of anti-splash products limit their application in anti-fog circumstances. Most notably, surfaces coated with anti-splash agents provide the greatest benefits when the following conditions obtain: (1) the surface is heavily wetted, because tiny water droplets will not run off of the substrate surface and will obscure rather than clarify the view; (2) when the only concern is the visibility of relatively large objects on the wetted side of the surface; and (3) when there is a large distance between the objects perceived on the "wet" side of the surface and the perceiver, such that refraction of light rays by irregularities in water droplet thickness is minor relative to the information content which passes through.
Numerous compositions have been proposed for anti-fog or anti-splash applications. U.S. Pat. No. 5,853,896 discloses a water-repellant composition consisting of a mixture of organosilane having the formula R.sup.1.sub.a SiX.sub.(4a). Anti-fog compositions include, among others, U.S. Pat. No. 5,804,612, which discloses a composition comprising a hydroxyl group containing polymer, an aluminum containing crosslinker, and a surface active agent containing hydroxyl and/or siloxane groups. U.S. Pat. No. 5,244,935 discloses a UV curable amide polymer. U.S. Pat. No. 5,075,133 discloses a crosslinked poly(vinyl alcohol) coating. U.S. Pat. No. 4,478,909 discloses a poly(vinyl alcohol)-silica system. U.S. Pat. No. 4,127,682 discloses a crosslinked poly(vinyl alcohol) coating. U.S. Pat. No. 4,467,073 discloses a coating based on poly(vinyl pyrroloidone). U.S. Pat. No. 3,933,407 discloses an acrylic siolxane system. U.S. Pat. No. 3,865,619 discloses a crosslinked carboxylic acid-acrylic acid ester coating.
In certain applications, such as, for example, dental mirrors, where both anti-fog and anti-splash properties are required, where accurate reflection of a patient's dental details is of paramount importance, and where non-toxicity is imperative, none of the foregoing compositions is completely satisfactory. None represent a total solution to the anti-fog/anti-splash application. Some require lengthy curing. Some are toxic. Many will immediately dissolve and wash off from a dental mirror in the water generated by high speed drills in current use. And none provide adequate visibility for viewing details in the operative field: the proximity of a dental mirror to the tooth diffuses the reflection and obscures tooth details necessary for precise dental work.